Trends in Data-Center Cooling

Increasing heat densities and a lack of thermal predictability within data centers are huge problems plaguing information technology (IT), facility, and data-center managers. These problems, along with spiraling energy costs and the drive to be “green,” have led to the adoption of new architecture and technology. The success of this new architecture and technology has led to its wide-spread acceptance and influenced current data-center-cooling trends. These trends include close-coupled cooling architecture, air containment, and variable-speed fan control. While key issues may vary among data-center managers, these trends quickly are emerging in data centers and have two common underlying themes: efficiency and cost.

The traditional approach to data-center cooling is based on room-level cooling, which utilizes down-flow computer-room air conditioners (CRACs) on raised-floor plenums to distribute cold air through perforated tiles. While this is an effective solution for heat densities of 2 to 3 kw per rack, facilities designed around this cooling architecture are overburdened because of an increase in heat densities caused by server technologies, such as blade and 1U servers. These technologies are driving heat densities in excess of 30 kw per cabinet. While not every data center is designed to this extreme, the trend toward high-density applications is occurring. The average design density today is 8 to 10 kw per rack and will grow to 20 kw per rack by 2010. Therefore, increased heat densities are driving the emergence of new cooling architecture.

To deal with increased heat densities, a common data-center-cooling design calls for cooling systems to be moved closer to the heat source. A closely coupled architecture can be implemented in a variety of ways. The two most common methods are rack- and row-based cooling. With these approaches, cooling units are integrated in a single rack or row of IT racks to provide cooling to complete zones or individual racks. They do not rely on expensive raised floors to distribute cold air to IT racks. Instead, they capture heat directly from racks or hot aisles, neutralize it, and deliver cool air back to servers. By virtue of close coupling, a cooling system can cool servers in a data center more efficiently by taking hot air directly from a hot aisle or the rear of racks. This increases cooling-unit capacity and reduces fan power to supply cool air to data centers. Because mixing is reduced greatly with this architecture, the cold air supplied back to servers can be delivered at much higher temperatures. With a closely coupled system, room-neutral-temperature air (typically between 68 and 72°F) is delivered back to adjacent racks. With an increase in supply temperature, coil temperatures can remain above dew point, preventing unwanted dehumidification and increasing sensible capacity.

Along with maintaining closely coupled cooling architecture, another trend concerns the isolation or containment of hot and cold air streams in data centers. Containment eliminates unwanted air recirculation and improves the overall efficiency of a closely coupled system. One example of a containment system is a zonebased approach that allows for containment of a hot aisle. Partitions are used to cap the top of a hot aisle and bridge the opening over the top of IT racks. Doors are used to seal the ends, creating a plenum containing heat that is captured and cooled by cooling units located in the row. Containment increases efficiency by further increasing the temperature of air returned to the cooling system, maximizing the available capacity.

Another noticeable trend in today's data centers is the use of variable-speed fan control. Growing data-center heat loads are increasing the power required for cooling. To help conserve energy, variable-speed fan control is being implemented. The traditional approach to data-center cooling relies heavily on the pressurization of raised-floor plenums to cool servers. Using variable-speed fans in this architecture requires sophisticated controls and baffling under floors to ensure even air distribution at less than full fan speed. The rise of closely coupled systems is enabling variable-fan-speed technology to be more practical. These systems monitor temperatures at the rack and row levels to control fan speed and balance cooling output to the thermal demand of servers.

As data centers become more dynamic with technology, such as virtualization, loads within them can shift. The use of variable-speed fans can reduce energy consumption by allowing cooling systems to rightsize themselves to the heat loads, ensuring well-balanced and efficient systems.

John Niemann is product-line manager for InRow cooling solutions for APC-MGE. He has 10 years of experience in the design, sale, and support of complex cooling solutions and information technology. He has a bachelor's degree in mechanical engineering from Washington University in St. Louis.